Yi Lu,Chen-Xi Yue,Li Zhang,Deqiang Yao,Ying Xia,Qing Zhang,Xinchen Zhang,Shaobai Li,Yafeng Shen,Mi Cao,Chang-Run Guo,An Qin,Jie Zhao,Lu Zhou,Ye Yu,Yu Cao
Precise regulation of intracellular phosphate (Pi) is critical for cellular function, with XPR1 serving as the sole Pi exporter in humans. The mechanism of Pi efflux, activated by inositol pyrophosphates (PP-IPs), has remained unclear. This study presents cryo-electron microscopy structures of XPR1 in multiple conformations, revealing a transmembrane pathway for Pi export and a dual-binding activation pattern by PP-IPs. A canonical binding site is located at the dimeric interface of SPX domains, and a second site, biased toward PP-IPs, is found between the transmembrane and SPX domains. By integrating structural studies with electrophysiological analyses, we characterize XPR1 as an IPs/PP-IPs-activated phosphate channel. The interplay among its TMDs, SPX domains, and IPs/PP-IPs orchestrates the conformational transition between its closed and open states.
XPR1 is emerging as the only known inorganic phosphate (Pi) exporter in humans, critical for Pi homeostasis, with its activity stimulated by inositol pyrophosphate InsP8 and regulated by neuronal scaffold protein KIDINS220. Our structural studies reveal that InsP8 specifically activates XPR1 in a stepwise manner, involving profound SYG1/PHO/XPR1 (SPX) domain movements. Each XPR1 subunit functions with four gating states, in which Pi permeates a constriction site via a “knock-kiss-kick” process. By contrast, KIDINS220 delicately stabilizes XPR1 in a closed conformation through multiple mechanisms, one of which involves trapping the XPR1 α1 helix—critical for InsP8 binding—within an interaction hub. InsP8 serves as a key to release KIDINS220’s restraint, reinforcing a “key-to-locks” mechanism to safeguard the stepwise activation. Additionally, our study provides direct structural insights into XPR1-associated neuronal disorders and highlights the evolutionary conservation and divergence among XPR1 orthologs, offering a comprehensive understanding of Pi homeostasis across species.
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